The rotator cuff muscles of the shoulder provide stability and motion at the glenohumeral joint. Rotator cuff disease is a degenerative condition that leads to significant shoulder pain, muscle atrophy, and tendon rupture, severely limiting upper extremity function. Rotator cuff repair to treat shoulder pain and restore strength is one of the most common orthopedic surgical procedures, however, rotator cuff tendon healing is unpredictable, with short to midterm failure rates ranging from 30- 94%. The majority of rotator cuff disease is related to chronic degeneration of the cuff tissues. Degenerative changes can lead to massive, irreparable rotator cuff tears through attritional changes in the tendons and irreversible fatty degeneration of the rotator cuff muscles. To date, most rotator cuff experimental studies have used acute injury and repair animal models (i.e., a healthy muscle and tendon is injured and then immediately repaired). These studies, while valuable, are only relevant to a small percentage of the rotator cuff cases seen clinically. In this proposal we seek to examine rotator cuff degeneration and repair using our newly developed, clinically relevant, animal model of rotator cuff disease. Understanding the process of rotator cuff degeneration and the potential for a degenerated cuff to heal will greatly enhance the clinician's ability to treat rotator cuff disease. Musculoskeletal tissues are highly sensitive to their mechanical environment. Clinical and experimental studies support the idea that decreased loading, either through tendon injury or denervation, leads to muscle and tendon degeneration. Rotator cuff tendon tears unload the rotator cuff muscles, leaving them vulnerable to deterioration and ultimately a chronically degenerated, irreparable rotator cuff. With regard to rotator cuff repair, the role of mechanical loading is more complex. In acute rotator cuff repairs, protective immobilization is beneficial to healing but complete removal of loading is detrimental to healing. Understanding the role of the mechanical environment for rotator cuff degeneration and repair of chronically degenerated cuffs will allow us to propose rehabilitation strategies to maximize muscle recovery and improve healing after surgical repair. Therefore, the overall objective in this study is to examine the role of the mechanical environment on rotator cuff degeneration and healing. Our approach will use a rat rotator cuff animal model to study cuff degeneration (Aim 1) and repair (Aims 3 and 4). We will compare results in our animal model to clinical data to validate that our basic science studies are relevant to the human condition (Aim 2).